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EMM Procedure 2. Detach
January 21, 2014 | By Netmanias (tech@netmanias.com)
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Page 2 of 7

 

 

 

2. Cases of Detach

 

A user uses LTE services after generating an EPS session and default EPS bearer through the initial attach procedures. In some cases, he may detach from the network once done using the services. In other cases, he may be detached by the network while still using services through the network, and becomes unable to stay connected to the network any more. 

 

Once a user is detached from the network, all the network/radio resources allocated to the EPS session and bearer established for the user are released. This release will delete the user’s MM context and EPS bearer that have been set to the EPS entities (UE and network nodes). At this time, the EMM state transits from Registered to De-Registered. If the user is properly detached, GUTI, a NAS-level user ID, and the security context that he used to access the network are kept valid in the UE and the MME, so that he can use the same in his next access to the network.

Detach can be triggered by UE or a network. Network-triggered detach is caused by either MME or HSS. Detach can be categorized as one of the following cases depending on where detach triggering is detected: 

 

1) Detach Case 1: UE-initiated Detach

UE can initiate detach: 

  • if UE is turned off
  • if a USIM card is removed from UE
  • if UE is attempting to use a non-EPS service (e.g. CS fallback, SMS, etc.)

2) Detach Case 2: MME-initiated Detach

MME-initiated detach can be further divided into explicit detach and implicit detach. In case of explicit detach, MME notifies UE of its intent to detach in advance by sending a Detach Request message, and informs the UE whether it has to attach the network again or not after detach. In case of implicit detach, however, the MME initiates detach procedures without notification (i.e. without sending a Detach Request message) because the UE is not capable of communicating with the MME. MME can initiate:

 

i) Explicit Detach

  • for an operator’s O&M (Operation & Maintenance) purposes
  • if re-authentication fails
  • if it cannot provide the resources allocated to a user

ii) Implicit Detach

  • if it is not able to communicate with a user because of poor radio link quality (e.g. radio link failure)

3) Detach Case 3: HSS-initiated Detach

HSS can initiate detach:

  • if the user profile provisioned in HSS is changed, and thus the one saved in MME also has to be changed
  • if an operator is trying to restrict access by an illegal UE (e.g. a stolen device) to its network

The next three chapters (III, IV and V) describe different detach procedures required in the three detach cases mentioned above. In all three cases, it is assumed a user is in EMM-Registered, ECM-Connected and RRC-Connected state before detach, and services are provided through the default EPS bearer only. Figure 1 illustrates what connections are established, and in what state UE and MME are in user/control planes before and after detach. Before detach, a default EPS bearer and its related control connections are established, and the user is in EMM-Registered, ECM-Connected and RRC-Connected. Then, after detach, the default EPS bearer and all the signaling connections are released, and the user enters EMM-Deregistered, ECM-Idle and RRC-Idle state.  

 

 

Figure 1. Connections and States before/after Detach

 

Page 2 of 7
Hector 2014-05-18 08:58:45
Hi guys, great information, congrats!
About the UE initiated detach reasons "UE is attempting to use a non-EPS service (e.g. CS fallback, SMS, etc.)". In this case, does the mobile sends explcitly a detach? (For CS Fallback, the mobile starts a detach procedure?)
Rahul Mishra 2015-10-16 23:56:00

Hi,

I've one question.

If UE initiates Detach with cause as Switch off, does the network stiil sends RRCConnectionRelease to UE.

yuvraj patil 2017-10-11 18:07:06

Hi,

when UE gets switch off, UE stops sending a periodic tracking area updates and N/W will read this situation as UE is not reachable and in this case, N/W will initiate detach procedure. 

Michael 2015-10-19 13:02:18

The MME transmits the UE context release command to eNB to release the UE-associated logical connection. Then, the eNB transmits the RRC connection release to the UE, releases the eNB resource, and transmits the UE context release complete to the MME to complete the call release procedure. 
An MME-initiated call release occurs in the following cases.
1. A procedure between UE and MME is completed (e.g. Detach).
2. A call in the source eNB is released due to successful handover.
3. A call in the target eNB is released due to cancelled handover.
4. The old logical connection is to be released when 2 logical connections are established for a specific UE (e.g. when UE duplication occurs in the MME).
5. The UE is not a CSG member of the CSL cell.

TianMing 2016-10-20 18:19:12

Hi, if ue combined attached to lte , if imsi detach, whether only deactive the cs

domain  security and authentication?

yuvraj patil 2017-10-11 19:26:52

Hi,

can anyone tell about PCC Rule Removal?

Sachin Vishwasrao 2018-01-20 21:02:39

After Step 16 in case of MME initiated Detach is the UE expected to ack the RRC Connection release message? I think since UE has released all the resources after sending the Detach Accept it cannot send the Ack. But in one of the operator specs it is expecting the RRC Connection release ACK. Please comment.

Tushar Waman 2019-01-29 18:27:27

Hi,

I would like to know, what happens when the battery is removed from the UE, how does the detach scenario play out ?

I believe the UE is not able to send any detach request when the battery is removed abruptly, but I would like to know how does the NW perform a detach in this case(if at all it perfoms a deatch). Tried reading the specification, but I could not find this Use Case. Any help would be appreciated. 

 

sasi.reddy@Ltts.com 2019-03-22 01:42:30

Not able to register since my company email id has "." Eg. sasi.reddy@Ltts.com

bala nagarjuna 2019-09-04 19:05:19

Hi Tushar,

if Battery is removed UE doesnt send detach request to Network. Network doesn't know status of UE thinks it is in IDLE.

 

here 2 things may let the Network about UE.

 

1. paging message sent by Network, there will be no RRC connection from UE so Network may think UE is in out of coverage. based on this Network can trigger internal detach.

2. There will be no Periodic TAU after perodic timer expiry, so Netowrk considers UE is in out of coverage and implicitly Detaches the UE

Inder Chand 2020-02-28 19:51:32

Why do UE get Detach type=3 and attach type = 2 in LTE detach and attach process


 

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